Interconnections and communications between computer circuitry is becoming increasingly important as information networks expand. Also, as personal computers become smaller and cellular telephone communications become more widely utilized, the integration of computer technology and wide area communication is expected to substantially increase. In addition to telephone communications, radio frequency communications are starting to play a more important role in communication interfaces between computer circuitry.
The use of radio frequency communications has been recognized by the federal government. The Federal Communications Commission (F.C.C.) has allocated two radio frequency bands for unlicensed use. The frequencies allocated by the F.C.C. are approximately 900 megahertz and 2.4 gigahertz.
Most computer circuitry is formed using Complementary Metal Oxide Silicon (CMOS) processes. CMOS circuitry has been clocked with frequencies up to 66 MHz and the current state of the art is about 100 MHz. Until now pushing CMOS circuitry to frequencies well beyond 100 MHz has not appeared feasible. For example, a junction capacitance of five picofarads at 100 MHz has an impedance of 320 ohms. Thus, CMOS circuitry has not appeared to be a candidate for use with the unlicensed radio frequencies listed above.
Specifically, the characteristics of Metal-Oxide-Silicon Field Effect Transistors (hereafter MOSFETs) create serious problems when used with radio frequencies. For the most part these problems are the result of inherent parasitic capacitances created at the junctions of the various layers of a MOSFET and the interconnections between MOSFET devices and other devices within a circuit.
To better understand the effects of the parasitic capacitances, consider a N-conductivity type MOSFET 100 (FIG. 1) that includes a source region 101 and a drain region 102 formed in a P-type silicon substrate 105. A silicon dioxide layer 104, sometimes referred to as an oxide layer, separates a gate 103 from an upper surface 105A of substrate 105.
FIG. 2 is a schematic representation of an electronically equivalent circuit 200 for MOSFET 100 that explicitly shows parasitic capacitances CRS, CGS, CGD, CRD, CTS and CTD.
CRS is the capacitance resulting from misalignment and overlap of gate 103 with respect to the source 101 diffusion. CRD is the capacitance resulting from misalignment and overlap of gate 103 with respect to the drain 102 diffusion.
The capacitances CTS and CTD are depletion-region capacitances at the reverse-biased pn junctions.
Capacitances CGS and CGD are inherent to the device and represent the flux linkages to the channel charge which gives rise to the basic operation of the device.
As noted above, capacitances CGS and CTD are the only capacitances theoretically inherent to the device, however, as a practical manufacturing limitation capacitances CRS, CRD, CTS, and CTD will also be inherent to any physically existing MOSFET.
The parasitic capacitances illustrated in FIG. 2 impede and prevent rapid voltage changes associated with radio frequency operation. Further, these parasitic capacitances effectively operate as part of a low pass RC filter in that the combination of the parasitic capacitances and the load resistance makes the device attenuate high frequency voltage changes. Additionally, the capacitance of the electrical connections between the various MOSFETs in a circuit and between MOSFETS and other circuit components is another source of parasitic capacitance and is often the most serious source of problems when attempting to amplify high frequency voltage signals.
Consequently, MOS circuitry and in particular CMOS circuitry has not generally been considered acceptable for use in radio frequency applications. As a result, large portions of the electro-magnetic spectrum have been thought to be useless for applications involving MOSFET devices.